Aggregate is a broad category of coarse particulate material used in construction, including sand, gravel and crushed stone. Aggregates are the most mined materials in the world. The term “sand” typically covers aggregate having a grain sand of between 0.075 mm and 4.75 mm while the term “gravel” typically covers aggregate having a grain size of between 4.75 mm and 76.2 mm. Aggregates are typically washed and graded on a combination of vibrating screens and hydrocyclones to produce washed aggregate products having a predetermined grain size or range of grain size.
A typical vibrating screen comprises a frame, defined by a pair of substantially parallel side walls interconnected by transversely extending bridging members, upon which is mounted a polyurethane deck having small openings or slots for water and undersize particles to pass through. The frame is typically mounted on a base via resilient linkages, and the frame (and thus the deck) is typically vibrated by means of a pair of counter rotating rotors defining eccentric masses, driven by one or more drive motors, to impart circular or reciprocating vibrating motion to the deck. Such screens can be used for grading and/or dewatering aggregate, oversize material passing over the deck of the screen to be collected from a downstream end of the screen while water and undersize material is collected in a sump of the screen for subsequent processing.
A hydrocyclone is a device used to separate particles in a liquid suspension based on the ratio of their centripetal force to fluid resistance. This ratio is high for coarse particles and low for fine particles. A hydrocyclone typically comprises a cylindrical section having an inlet for supplying a feed slurry into the hydrocyclone tangentially, and a conical base. Outlets are provided at upper and lower ends of the hydrocyclone. Underflow, containing the coarser fraction, passes out of the lower outlet while overflow, containing the finer fraction and most of the water, passes out of the outlet at the upper end of the hydrocyclone.
Most aggregate washing and grading plants are very large, including different stages comprising multiple grading and dewatering screens and hydrocyclones, and typically require a large volume of water to fluidise the material in each stage of the process and to transfer the material between different stages of the process. Such plants require considerable installation time and are not readily moveable once installed on a site.